1. Field of the Invention
The present invention relates to an electronic flash apparatus to be connected, in use, to a camera.
2. Related Background Art
As a terminal for electrically connecting a camera with an electronic flash unit, there is already known a terminal, known as X-contact, for instructing start of flash emission from the camera to the electronic flash unit.
Also there are known a ready terminal for sending a predetermined signal from the electronic flash unit to the camera when a main capacitor of the electronic flash unit is charged to a predetermined voltage; a stop terminal for sending a signal from the camera for terminating the flash emission of the electronic flash unit; and a terminal for mutually communicating the performance of the camera and the electronic flash unit, such as the possibility of TTL light control.
In recent systematization of the camera and the electronic flash unit, various data on the flash unit including not only the above-mentioned charge signal but also a maximum light amount signal and data signals for light amount control and various data on the camera such as the diaphragm aperture and the film sensitivity become necessary for automation and diversification of flash photography, and such data have to be exchanged between the camera and the flash unit.
Consequently new terminals for transmitting such data may be added.
A method of multiplexing the signals transmitted by each terminal by adding digital signals to the conventional signals, thereby transmitting a large amount of data, is disclosed in U.S. Pat. No. 4,695,148, and a method of displaying such data is disclosed in U.S. Pat. No. 4,573,786.
Also there is known a method of photographing with flash, called daylight flash photography. Since the background of the object is of a high luminance, the photographing condition is apt to be influenced by the background illumination, and the amount of flash light emission from the flash unit may be terminated before reaching the appropriate amount, due to the influence of the background illumination for example if the measurement of luminance of the object is initiated prior to the start of flash light emission.
In order to prevent such phenomenon, U.S. Pat. No. 4,695,148 discloses a method of starting the measurement of luminance of the object simultaneously with the flash light emission from the electronic flash unit. This method also enables the transmission of digital signals, and enables sending information on the timing of starting the luminance measurement.
However such conventional technologies as explained above are associated with the following drawbacks.
In the apparatus for daylight flash photography, disclosed in U.S. Pat. No. 4,695,148, the electronic flash unit is unable to distinguish the current status thereof if it is not connected to the camera or if the power supply of the camera connected thereto is turned off.
More specifically, even when the electronic flash unit is connected to a camera capable of data transmission, the flash unit cannot identify whether the power supply of the camera is turned off or it is connected to a camera without data transmitting function.
This drawback is likewise present in the technology disclosed in U.S. Pat. No. 4,573,786. In the apparatus disclosed in that Patent, a display is made on the flash unit on the data transmitted from the camera, or on the data determined by another method if data transmission is not made from the camera. However, if the power supply of the camera connected to the flash unit is turned off or if the flash unit is not connected to the camera, the flash unit is unable to identify the proper display to make and may show an erroneous display.
Also certain cameras with an accessory shoe are so constructed that the electrical connection with the electronic flash unit is made through a connector other than the accessory shoe. In such cameras, the accessory shoe is generally made of metal, and, when the electronic flash unit is mounted on such accessory shoe, the contacts, provided in the flash unit for electrical connections with the camera, are all grounded.
Consequently the electronic flash unit becomes inoperable, but the flash unit is unable to understand such state and is therefore unable to provide an alarm.
In the apparatus disclosed in U.S. Pat. No. 4,695,148 in which the signal exchange between the camera and the flash unit is conducted by multiplex digital signals and the timing of start of light measurement is transmitted from the flash unit, there is required a time for distinguishing the sequence for exchange of digital signals from the sequence for transmitting the timing of start of light measurement, since both sequences are switched by the duration of L-level state of a signal terminal. Particularly the transmission of the timing of start of light measurement is conducted when the shutter is released, so that the time from the release of shutter to the light emission of the flash unit becomes extended. Consequently there is required a significant time lag from the shutter release by the operator to the actual photographing operation.
Also, the sequence may be mistaken if the response of the flash unit is delayed, as two sequences are distinguished only by time.
Furthermore the exchange sequence of the digital signals is unilaterally initiated by the camera, and the digital signals are transmitted irrespective of the connected flash unit. Consequently if for example a flash unit with auxiliary light for auto focusing as disclosed in U.S. Pat. No. 4,804,991 is mounted, the auxiliary light becomes unnecessarily turned on.
Also in case the number of flash units is increased under TTL light control as disclosed in Japanese Laid-open Patent Sho 58-62633, the stop terminals are connected in parallel. Thus an increase of the current in the stop terminals gives rise to an increase in the current flowing into the camera in the photographing operation with an increased number of flash units, and the camera may become unable to output a flash stop signal, for terminating the flash emission, by because the voltage may drip to the L-level through sinking of the current in the stop terminal, as the current exceeds the sinking ability.